In an oil and gas well, cement between the casing and borehole is designed to provide zonal isolation in the wellbore. However, liquid contaminants or defects created by volumes of inadequate density might lead to failed isolation, and fixing these defects can be expensive and difficult.
Therefore, methods and devices for detecting defects in the cement and analyzing the quality of the cement are important to the oil and gas industry. One technique commonly used is acoustic logging. Acoustic logging detects and assesses the cement behind the casing by measuring the acoustic impedance of the cement bonded to the casing. However, conventional acoustic tools are subject to important limitations. For example, acoustic decoupling can be caused by the presence of a micro-annulus or a shear film coating in the casing. A micro-annulus may allow the casing to move, thus breaking the bond between the cement and the casing. A micro-annulus may be partial or, in some instances, it may extend around the entire casing circumference, thereby possibly resulting in undesirable fluid communication between zones.
Another technique applied involves the use of ultrasonic imaging tools, which are based on pulse-echo techniques. However, such tools are limited when heavy mud or thick casing is used since the reflected signals decay very quickly. For example, the typical upper limit of the casing thickness allowable in pulse-echo ultrasonic measurement is about 0.59 inches (about 15 mm). Therefore, this technique is typically only suitable if the volume of interest is very close to the casing and strongly bonded to the reflective surfaces. Otherwise, such defects may go undetected
Therefore, there is a need in the art for alternative improved techniques in which to detect defects behind the casing.